As is generally known in the glass producing art, it is desirable to supply the melting furnaces with homogeneous meltable glass mixtures. The homogeneity of the mixtures to be supplied to the furnaces is important since non-homogeneous mixtures cause serious melting problems. This is so because the ingredients which facilitate the melting process should be uniformly distributed throughout the mixture in order to avoid problems.
As is known, glass-producing mixtures have a tendency to de-mix or separate during transportation to the glass melting furnace. Several procedures have previously been suggested with a view to counteracting this de-mixing tendency of the mixtures. It has thus been proposed to shorten the distance between the location where the mixture is prepared and the furnace. Further, it has been attempted to moisten the glass-producing mixture. Experience has, however, demonstrated that these prior art methods do not significantly reduce the de-mixing tendency, particularly in respect of fine particles. It should also be considered that glass-producing mixtures which comprise fine particles have the disadvantage that the heating gases during the melting procedure in the furnace tend to carry along the fine particles which thus become entrained in the gases passing through the furnace. These particles then deposit on the brickwork of the furnace on which they have a strongly corroding effect. In order to avoid such a dusting in the furnace or melting vessel, the heat supply in the form of the heating gases has thus to be limited which in turn results in an output reduction of the melting furnace.
With a view to overcoming the problems caused by the de-mixing of the glass-producing mixture and the dusting in the furnace as referred to above, it has previously been suggested to form granules, pellets or briquets from the mixture prior to its entry into the melting furnace. Thus a number of granulating, pelletizing and briquetting procedures have been suggested for this purpose. It will be appreciated that glass-producing mixtures in piece or granule form--hereinafter sometimes referred to as discrete pellets--cannot readily de-mix during shipping and storage and moreover do not dust within the furnace. An additional advantage of such discrete pellets as distinguished from glass-producing mixtures in fine particle form is that the pellets have a greater density and thus permit improved heat transfer.
However in considering the above it has to be realized that customary glass-producing mixtures as they are used in the industry can only successfully be agglomerated into discrete pellets or granules if they contain a predetermined amount of fines, to wit very fine particles. For this reason and in order to form the desired discrete pellets it was first necessary to grind or comminute the ingredients of the mixture, particularly the sand, so as to obtain the necessary amount of fines. This in turn required considerable investment in respect of plant and apparatus and generally rendered the operating costs more expensive. The grinding, moreover, due to mechanical friction and abrasion action in respect of the walls of the grinding apparatus caused an undesired increase in the iron content of the glass-producing mixture. In order to avoid the comminution or grinding procedure, German Offenlegungsschrift No. 2,030,011 suggests to use a glass-producing mixture which contains caustic sodium hydroxide as the sodium component with at the most 15 percent by weight of water. The presence of this sodium hydroxide renders the mixture capable of agglomerization without prior grinding. However it will be realized that the shipping, storage and dosing of caustic soda melts of such high concentration causes considerable practical problems since such caustic sodium hydroxide melts are highly corrosive and have a solidification point above 180.degree.C.
German Offenlegungsschrift No. 2,108,656 furthermore proposes to render glass-producing mixtures capable of granulation by adding market available sodium hydroxide solution of only 50 percent concentration and quicklime as the calcium component of the mixture. Granules or pellets obtained in this manner do not possess the disadvantages of glass-producing mixtures in fine particle form. The same applies to briquets formed from glass-producing mixtures with the aid of aqueous sodium hydroxide. However, such granules, pellets or briquets have in turn the disadvantage that the sodium hydroxide contained therein is hygroscopic. When the solid pieces, be they in granule, pellet or briquet form, are stored for prolonged periods of time, the hygroscopic nature of the sodium hydroxide causes disintegration of the solid pieces unless they are not subsequently treated with carbon dioxide so as to convert the sodium hydroxide into sodium carbonate. Such after-treatment with CO.sub.2 results in storage-resistant solid pieces (granules, pellets or briquets). However during the subsequent melting in the glass-producing furnace, the carbon dioxide is again liberated from the sodium carbonate. This in turn requires refining of the glass melt with expensive refining agents. These refining agents, upon ultimate discharge from the furnace have a polluting effect on the ambient environment. Moreover the refining treatment with such agents results in annealing losses and negatively affects the heat passage through the solid pieces. These disadvantageous effects decrease the melting output of the furnace and result in increased expenditure in respect of operational efforts and energy.
In discussing the prior art, reference is also had to U.S. Pat. No. 3,542,534 which discloses a procedure wherein components of the glass-producing mixtures are first comminuted to form a fine particle mass which is thereafter granulated in the presence of sodium hydroxide whereafter the granulated mixture is subjected to a sintering procedure at a temperature of 200.degree. to 700.degree.C in order to impart the granules with sufficient stability. Such granules or solid pieces however do not result in any improved melting effect in the glass melting furnace since no eutectic mixtures with the sand are formed. Further also in this procedure, the aqueous sodium hydroxide is converted by the flue gases into sodium carbonate which, upon melting in the furnace, liberates carbon dioxide.
U.S. Pat. No. 3,503,790 discloses a procedure wherein an intermediate product for glass-producing batches in the form of a freely flowing powder is obtained. This powder is formed from sand particles which are partially coated with sodium metasilicate by heating the sand with individual sodium hydroxide particles of the same particle size order at temperatures of 300.degree. to 450.degree.C. However this procedure does not result in a glass-producing mixture in the form of discrete pieces or pellets so that this prior art procedure is not comparable with the inventive procedure. This prior art procedure does not solve the problem of de-mixing of the individual ingredients of the glass-producing mixture nor is it concerned with the dusting in the glass melting furnace.
In connection with the discussion of the prior art reference is also had to U.S. Pat. No. 3,760,051.